The present invention relates to a plasma torch of the type composed of a central electrode and nozzle which concentrically surrounds the electrode.
During the operation of plasma torches, a stable electric arc column must form between the electrode and a counterelectrode. The central electrode is surrounded by the nozzle and is composed of a single electrode or of a centrally disposed auxiliary electrode and a primary electrode which concentrically surrounds the auxiliary electrode. The counterelectrode is provided, for example, in the form of a bath of molten metal. The desired stability of the arc and thus the efficiency and economy of operation of a system operated with such a plasma torch can here be adversely affected to a considerable degree by parasitic arcs. Such parasitic arcs burn parallel to the primary arc and include, in particular, the lower edge of the outer burner or nozzle jacket and the outer region of the frontal face of the nozzle in the current flow.
The formation of parasitic arcs involves three contiguous current paths, with the first current path being formed by an internal short circuit arc which electrically bridges the relatively short path between the electrode and the nozzle; the second current path is the metallic conductor formed by the nozzle; and the third current path is formed by a double arc burning from the outer torch or nozzle jacket or the outer region of the frontal face of the nozzle to the counterelectrode. Particularly when high intensity, liquid cooled plasma torches are used in hot furnaces, e.g. for melting scrap, such parasitic arcs may develop and may cause the premature failure of the plasma torch, primarily in that the frontal nozzle jacket or the nozzle frontal face burns through, but also due to extensive wear of the torch electrode.
To counteract this phenomenon, it is known to reduce the current intensity of the primary arc, or to at least limit it so as to thus protect the nozzles against burning through and to prevent excess wear of the electrode. See in this connection German Auslegesschrift No. 2,140,241, German Pat. No. 2,541,166, German Offenlegungsschrift No. 2,951,121 and East German Pat. No. 97,364.
Aside from the fact that in the stated cases a considerable amount of apparatus is required to detect the parasitic arcs and to reduce or limit the primary arc current, the appearance of parasitic arcs and their negative effects are merely reduced, but not reliably prevented. Moreover, measures for combatting parasitic arcs always require that the power be drastically choked off or even that the torch be turned off.
It is further known to cover the outer jacket of the nozzle with an electrically conductive layer having a high melting or sublimation point (see German Offenlegungsschrift No. 3,307,308). This layer, which may be composed, for example, of solid graphite, wears slowly and continuously under the effect of parasitic arcs and thus counteracts premature and sudden wear of the actual metallic torch nozzle. However, such protection is not only limited in time, it is also unsuitable to compensate for the poor efficiency of the system caused by the parasitic arcs. Moreover, this known protective measure does not provide protection for the central electrode since it is attacked by the internal short circuit arc.
It is also known from U.S. Pat. No. 3,147,329 to provide the frontal face of the nozzle with a heat-resistant lining. Although this provides a certain local protection for the nozzle, the generation of parasitic arcs is at most made more difficult thereby, but is not effectively prevented.